Japanese Patent Laid-Open No. 2006-256560 describes a power output unit in which a motor as a prime mover is connected to a transmission, and in which a torque is delivered to a right derive wheel and a left drive wheel through a differential gear. If the motor generates a large drive force when stopping or when rotated at a low speed, one of coils of specific phase would be heated by overcurrent. Such phenomenon is called a “single phase lock”. In order to avoid the single phase lock, according to the teachings of Japanese Patent Laid-Open No. 2006-256560, an engagement device such as a brake of a transmission is controlled in such a manner as to cause a slip, and the motor is controlled in such a manner as to generate a target drive torque.
Japanese Patent Laid-Open No. 2011-231857 describes a driving device having a motor-generator serving as a prime mover. In the driving device, the motor-generator is connected to a torque converter having a normal-close type lockup clutch that is normally applied to transmit a torque.
In the power output unit taught by Japanese Patent Laid-Open No. 2006-256560, the motor is rotated while reducing a torque transmitting capacity of the engagement device to avoid the single phase lock so that the power of the motor is partially consumed in the form of friction heat. However, if the motor is thus rotated while causing a slip in the engagement device, the engagement device would be damaged by the friction heat. Such damage on the engagement device may be limited by reducing friction of the engagement device. However, if the friction of the engagement device is reduced, a rotational speed of the motor cannot be raised to avoid the single phase lock. Otherwise, the engagement device is almost released and hence a drive torque cannot be transmitted sufficiently to propel the vehicle. In the driving device taught by Japanese Patent Laid-Open No. 2011-231857, a motor-generator is also used as a prime mover. However, Japanese Patent Laid-Open No. 2011-231857 does not disclose a possibility of the single phase lock and a means to avoid the single phase lock.